Tunnel magnetoresistance (TMR) sensors are frequently employed as magnetoresistive sensors in the magnetic heads of hard disk drives. As the areal density of hard disks approaches 1 Tb/in2, it is desirable to reduce the magnetic reader gap spacing to about 25 nm or below. To do so will require a thinner TMR sensor stack, however. Many readers have a free layer (FL) about 60-80 nm in thickness, in order to achieve high TMR ratio (dR/R) and lower magnetostriction. This FL design involves a thin first magnetic layer acting as an interface layer that has a high spin polarization and a high positive magnetostriction. To counteract the high positive magnetostriction of the first magnetic layer and achieve a desired net low magnetostriction (e.g., below 2×10−6) for the FL, a second, counteracting, magnetic layer with a negative magnetostriction may also be used.
One approach to providing a counteracting second magnetic layer involves using a nickel iron (NiFe) alloy with a Fe composition less than 10%, due to the low magnetic moment and low spin polarization degree of this material. In such an arrangement, a thick second magnetic layer and a thick first magnetic layer may be used to provide a high dR/R as well as a low magnetostriction. In addition, Ni-rich NiFe alloys have a high Gilbert damping constant, which contributes to a magnetic noise induced by a vibration of grain structures. Therefore, there is a need for a free layer structure that has a reduced overall thickness and a low Gilbert damping constant.